Capacitors for use in electronic instruments such as cellular phone and personal computer are demanded to have a small size and a large capacitance. Among these capacitors, a tantalum capacitor is preferred because of its large capacitance for the size and good performance. In this tantalum capacitor, a sintered body of tantalum powder is generally used for the anode moiety. In order to increase the capacitance of the tantalum capacitor, it is necessary to increase the mass of the sintered body or to use a sintered body increased in the surface area by pulverizing the tantalum powder.
The method of increasing the mass of the sintered body necessarily involves enlargement of the capacitor shape and cannot satisfy the requirement for downsizing. On the other hand, in the method of pulverizing tantalum powder to increase the specific surface area, the pore diameter of the tantalum sintered body decreases or closed pores increase at the stage of sintering, as a result, impregnation of the cathode agent at the later step becomes difficult. One of studies for solving these problems is to fabricate a capacitor using a sintered body of a material powder capable of giving a dielectric constant larger than that of tantalum. As such a material giving a larger dielectric constant, niobium and titanium are known.
JP-A-55-157226 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses a method for producing a sintered device for capacitors, where agglomerated powder or niobium fine powder having a particle size of 2.0 μm or less is molded under pressure and then sintered, the molded and sintered body is cut into fine pieces, a lead part is joined therewith and these are again sintered. However, the details on the properties of the capacitor are not described in this patent publication.
U.S. Pat. No. 4,084,965 discloses a capacitor using a sintered body of niobium powder of 5.1 μm obtained by hydrogenating and pulverizing a niobium ingot. However, the capacitor disclosed has a large leakage current (hereinafter sometimes simply referred to as “LC”) value and is of little practical use.
U.S. Pat. No. 5,242,481 discloses a production method where the oxygen content in a niobium powder, a tantalum powder or a niobium and tantalum alloy powder is reduced to 300 ppm or less by using a reducing agent such as metal magnesium. However, this patent publication does not describe a capacitor using these powders.
U.S. Pat. No. 6,171,363 discloses a production method where a metal or an alloy of tantalum, niobium, titanium, molybdenum, tungsten, vanadium, zirconium or hafnium is produced from an oxide of tantalum, niobium, titanium, molybdenum, tungsten, vanadium, zirconium or hafnium by reducing the oxide using a reducing agent such as gaseous magnesium or calcium and where a niobium-tantalum alloy, a niobium-titanium alloy and a tantalum-titanium alloy are described as a capacitor material substituting tantalum. However in Examples, only tantalum or niobium is used but a case of using a niobium alloy is not described and the performance of capacitor is not described either.
WO00/67936 discloses a production method where a metal or an alloy of tantalum, niobium, titanium, molybdenum, tungsten, vanadium, zirconium or hafnium is produced from an oxide of tantalum, niobium, titanium, molybdenum, tungsten, vanadium, zirconium or hafnium by reducing the oxide using a reducing agent such as gaseous magnesium or calcium. This patent publication discloses an example of niobium-tantalum alloy and states that when a niobium-tantalum alloy containing 15 atom % of tantalum is used, the thickness of the dielectric film per unit voltage becomes smaller than that when using niobium alone and thereby, the capacitance is increased. However, only tantalum or niobium is used in Examples and a case of using a niobium alloy is not described and the performance of capacitor is not described either.
JP-A-10-242004 discloses a technique of improving the LC value by, for example, nitriding a part of niobium powder. However, when producing capacitors having high capacitance from a niobium sintered body using the niobium powder having a small particle size, some capacitors exhibit a peculiarly large LC value.
JP-A-11-329902 (U.S. Pat. No. 6,215,652) discloses a niobium solid capacitor reduced in the change of electrostatic capacitance between before and after the reflow step at the mounting of parts. However, the capacitor disclosed has a capacitance as small as 2 μF and there are not disclosed high-temperature property with respect to the capacitance, which is described later, heat resistance and appearance frequency of defective/non-defective units with respect to LC.
These conventional niobium capacitors fail in fully satisfying all of capacitance, high-temperature property and heat resistance property and are not used in practice or even if used in practice, their use is very limited.
The ratio (C-C0)/C0 of the initial capacitance C0 at room temperature to the capacitance C after a capacitor is left standing for 2,000 hours while applying a voltage in an atmosphere of 105° C. and then returned to room temperature is defined as the high-temperature property. When a sintered body is electrolytically oxidized and then combined with a counter electrode to produce a capacitor, the high-temperature property of tantalum capacitors using a tantalum sintered body usually falls within ±20%, however, the high-temperature property of some niobium capacitors using a conventional niobium sintered body does not fall within ±20%.
The heat resistance property is expressed, as a measure therefor; by the number of units showing a leakage current value (LC value) of 0.05 CV (a product of capacitance and rated voltage) or less when 50 capacitor units are manufactured and connected to a previously prepared substrate in a reflow furnace and then measured on the leakage current. The temperature at the exterior terminal part of a capacitor at the time of charging the substrate into the reflow furnace is kept at 230° C. for 30 seconds per charging and the number of operations of charging the substrate is 3. When a sintered body is electrolytically oxidized and then combined with a counter electrode to produce a capacitor, the number of capacitor units having a heat resistance property of 0.05 CV or more is usually 0/50 units in the case of capacitors produced from a sintered body using a tantalum powder, whereas sometimes capacitor units having a heat resistance property exceeding 0.05 CV appeared in the case of capacitors produced from a sintered body using a conventional niobium powder.
The niobium sintered body is inferior to a tantalum sintered body in the stability of oxide dielectric film. This difference outstandingly comes out at high temperatures. Many reasons are considered therefore but as one reason, it is presumed that due to difference between the composition of oxide dielectric film and the composition of niobium sintered body electrode, heat distortion occurs at high temperatures and thereby, the deterioration of oxide dielectric film is accelerated.
As such, capacitors using a niobium sintered body must be rated low in the reliability at room temperature and the service life thereof is sometimes judged defective. Therefore, their use in practice is limited.